Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 261600

Area Forecast Discussion
National Weather Service Medford OR
900 AM PDT Fri May 26 2017

The forecast has been updated to account for the presence and
expected progression of marine layer fog and stratus along and
near the coast this morning into this afternoon. BTL


Extensive fog and stratus exists along the coast this morning and
fairly far up the coastal river valleys. GOES 16 visible
satellite imagery indicates northerly flow west of Florence and
several stratus eddies offshore south of there that should cause
the marine layer to mostly dissipate along the coast north of Cape
Arago today. However, the immediate coast will likely be plagued
by the marine layer south of there. Confidence with the details
of the visible marine layer is moderate. Otherwise, it will be a
mild and dry day across the forecast area. BTL


.AVIATION...26/06Z TAF CYCLE...IFR ceilings along the coast
north of Cape Blanco will continue into the morning, then should
burn off by afternoon. To the south, a surge of south winds and
IFR/LIFR fog is expected to move up the Curry coast, reaching KBOK
this morning and pushing up to Cape Blanco by this afternoon. This
surge will likely spread north of the cape later today, making it to
KOTH around 08 UTC this evening.

All other areas will remain VFR through the TAF period. -BPN


.MARINE....Updated 200 AM Friday 26 May 2017...North winds and
steep seas will gradually decrease today, then by this afternoon,
southerly winds are expected to develop along the coast near
Brookings. South winds and fog will then push northward, reaching
Port Orford by this evening, then rounding Cape Blanco and reaching
Cape Arago tonight. Light south winds through the weekend may swing
onshore at times, and fog is likely through at least Sunday morning.
A weak thermal trough is then expected to develop Sunday into
Monday, and may produce periods of  small craft advisory level winds
and seas south of Cape Blanco early to mid week. -BPN


.PREV DISCUSSION... /Issued 243 AM PDT Fri May 26 2017/

SYNOPSIS...An upper trough drops through the Great Basin today,
then a strong ridge builds over the PacNW and brings mostly sunny
and very warm to hot weather for the weekend and into Monday. A
Pacific trough brings a chance for showers and thunderstorms later
Monday and Tuesday followed by cooler weather for the most of the
rest of next week.

DISCUSSION...Stratus has move into portions of the coast this
morning, and stratus will gradually increase today and especially
tonight as a coastally trapped wind reversal spreads from south
to north along the coast. An upper trough dropping through the
Great Basin produce some clouds over the east side last evening,
but those have generally dissipated. We will probably see some
increase in mid cloud or some cumulus buildups this afternoon as
the back edge of the trough slides through, but that should be it.
For the rest of us today, it will be hard to notice much
difference from yesterday.

As the trough departs tomorrow, a ridge of high pressure builds
over the Pacific Northwest, which will bring hotter weather
through Monday. Temperatures will be 15-20 degrees above normal
for most areas inland from the coast during the period.
Instability will gradually increase, but there will not be enough
moisture or forcing to worry about thunderstorms until Sunday at
the earliest. Even Sunday the conditions do not look that
favorable, generally due to a lack of a triggering mechanism, but
we already had a slight chance for storms over the Cascades, and I
can`t rule it out, so they remain in the forecast.

Monday looks more impressive to me with respect to thunderstorm
potential. Instability becomes pretty extreme Monday with LIs -4
to -8, CAPES over 2000 j/kg, and mid level lapse rates over
7.5C/km from the Siskiyous into the Cascades per GFS - NAM will
likely be worse once it goes out that far. At the same time, PWAT
surges to over an inch on the west side, there is a slight
increase in upper jet dynamics, and there are even hints at a
shortwave to give all that juice the required kick. The weakest
link certainly looks like trigger, but the mountains themselves
often act as their own trigger, so Monday should have some storms.
Monday`s pops have been increased in the mountains and expanded
out into the northern Klamath, which is a favored corridor for
thunderstorms in this kind of scenario. While convection Monday
is unlikely to be organized, the models certainly show enough
instability to warrant keeping a close eye on it.

By Tuesday, the incoming trough will have probably stabilized
things enough to preclude any thunderstorms from the Cascade crest
westward, but thunderstorms remain possible on the east side.
Cooler weather with some chance for showers is expected Wednesday
and Thursday as the trough moves inland. The extended forecast
covered this well, so few changes were made. -Wright




Pacific Coastal Waters...None.


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